An oscillatory type (oscillatory wave) actuator includes an oscillator that oscillates driving vibrations in a ring-shape, prolate-ellipsoid-shape, or rod-shape elastic member in response to an electrical signal, such as AC voltage, applied to an electromechanical energy conversion element, such as a piezoelectric element. An example of the oscillatory type actuator suggested heretofore is an oscillatory wave motor in which an oscillator moves relative to an elastic member in pressure-contact with the oscillator.
A general structure of a ring-shape oscillatory wave motor is described below as an example.
A ring-shape oscillatory wave motor includes a piezoelectric material that has an inner diameter and an outer diameter such that the entire perimeter equals to an integral multiple of a particular length λ. A plurality of electrodes are disposed on one surface of the piezoelectric material and a common electrode is disposed on the other surface of the piezoelectric material to form a piezoelectric element.
The plurality of electrodes include two drive phase electrodes, a detection phase electrode, and a non-drive phase electrode. Electric fields of opposite directions are alternately applied at a λ/2 pitch to respective drive phase electrode portions of the piezoelectric material to conduct a polarization treatment. Accordingly, the polarity of expansion and contraction of the piezoelectric material with respect to the electric field in the same direction is reversed every λ/2 pitch. The two drive phase electrodes are spaced from each other by a distance equal to an odd multiple of λ/4. Usually, a non-drive phase electrode is formed in this gap portion to prevent the piezoelectric material in this portion from vibrating, and short-circuited with the common electrode via short-circuiting wires or the like.
The detection phase electrode is an electrode for detecting the vibrating state of the piezoelectric material. A strain generated in the piezoelectric material in the detection phase electrode portion is converted into an electrical signal corresponding to the piezoelectric constant of the piezoelectric material and output to the detection phase electrode.
A stator can be formed by connecting a wire for inputting and outputting power to this piezoelectric element and bonding a diaphragm composed of an elastic material to the piezoelectric element. When AC voltage is applied to one of the drive phase electrodes of the stator, a standing wave having a wavelength λ occurs throughout the entire perimeter of the diaphragm. When AC voltage is applied only to the other drive phase, a standing wave occurs in a similar manner but the position of the standing wave is rotationally shifted in the circumferential by λ/4 with respect to the standing wave mentioned earlier.
A ring-shape oscillatory wave motor can be formed by bringing a ring-shape elastic member into pressure-contact with a surface of the stator opposite to the diaphragm so that the ring-shape elastic member can serve as a rotor.
Another type of oscillatory wave motor is an oscillatory wave motor in which electrodes and a diaphragm are attached to inner and outer sides of a ring-shape piezoelectric material and a rotor is in pressure-contact with the inner or outer side of the piezoelectric material. This type of motor can be driven by the rotation of the rotor caused by expansion and contraction (vibration) of the piezoelectric material.
When AC voltages having the same frequency and a time phase difference of π/4 are applied to the respective drive phase electrodes of the oscillatory wave motor, standing waves become combined, and a travelling wave (wavelength λ) of bending vibrations travelling in the circumferential direction occurs in the diaphragm.
During this process, the points that lie on the rotor-side of the diaphragm undergo a type of elliptical motion and the rotor rotates due to the frictional force from the diaphragm in the circumferential direction. The direction of rotation can be reversed by switching the phase difference between the AC voltages applied to the respective drive phase electrodes between plus and minus.
A driving control system capable of controlling the rotation speed can be formed by connecting a control circuit to the oscillatory wave motor. The control circuit includes a phase comparator that compares the phases and outputs a voltage value corresponding to the result of comparison.
When an oscillatory wave motor is driven, an electrical signal output from the detection phase electrode is input to a phase comparator along with an electrical signal applied to the drive phase electrode. The phase comparator outputs phase difference so that the degree of deviation from a resonant state can be detected. The data is used to determine the electrical signal applied to the drive phase electrode and to generate a desired travelling wave so that the rotation speed of the rotor can be controlled.
However, the value of voltage output from the detection phase electrode is usually larger than the input upper threshold voltage value of the phase comparator. Accordingly, for example, the oscillatory wave motor control system disclosed in PTL 1 provides a mechanism (step-down circuit) between the detection phase electrode and the phase comparator to decrease the voltage to a logic level.